Effect Of Agricultural Practice On Reactive Nitrogen Pool In Soil And Environment

Man promotes greatly nitrogen immobilization and cycle worldwide by agricultural practice, which leads to serious environment pollution at the same time. People concerned about its effect on environment owing to more and more environment pollution after reactive nitrogen, an important nitrogenous form which can be utilized directly by creature on Earth, has been used broadly in agriculture practice due to its contribution to food production to raise people. Nitrogenous fertilizer is a source of reactive nitrogen in soil in agricultural ecosystem, and its fate was crop uptake, residue in soil and losses by possible ways after it is input into soil. Nitrate mostly affects environment among residue of nitrogenous fertilizer in soil. With the development of agriculture production, a large amount of nitrate is accumulated in soil profile after crop harvest derived from superfluous nitrogenous fertilizer application for crop yields. Some of residual nitrate may be taken and used by next crop, some may be leached into deeper soil layer, even superficial groundwater, and others may be transformed into N₂O or N₂ and emitted into atmosphere from soil. Nitrate leached is responsible for nitrate contamination in groundwater, and N₂O is one of gases leading to warm climate worldwide from denitrification. Unlike intensive agriculture ecosystem, natural ecosystem, such as forests and grasslands, keep inherent production without people attending. Few reports are found to research effect of agriculture practice on reactive nitrogen pool in soil in agriculture ecosystem and environment derived from change of reactive nitrogen pool according to the law of nitrogen cycle and transformation in natural ecosystem. Therefore, natural ecosystem (including forest and grassland) without people attending was selected as the reference to investigate the change of some important parts (NH₄⁺-N, NO₃^-V-N, mineralized N, soil microbial biomass N and nonexchangeable ammonium) in reactive nitrogen pool in agricultural ecosystem (including field, vegetable greenhouse and the field under the condition of long term fertilization), due to agricultural practice, reason of change and possible measure to relieve the change, meanwhile to research mineralization and nitrification potential of reactive nitrogen pool in soil, and thereby to estimate the possiblitily of nitrate of different soil in different ecosystems, the mechanic of reactive nitrogen transformation and environment effect so as to offer the reliable reference to restore polluted environment derived from nitrogen. The main results are as followed:Under the condition of farming, fertilization and irrigation, etc., NO₃^-V-N accumulation insoil profile in all agricultural ecosystems in our experiment was fairly high. Among different ecosystems, NO3"-N accumulation in soil in vegetable greenhouse was the highest and came to 717kg N hm'2 from the mean of five sampling areas, which was three times as many as annual nitrogenous fertilizer application on average farmland. The values in black soil (n=10), chernozem (n=ll) and Baijiang soil (n=5) were 78.3 kg hm"2, 124.6 kg hm"2 and 20.6 kg hm"2, respectively. The results indicated that NCV-N accumulation in farmland soil profile from different soil groups had been greatly higher compared with those in forest and grassland, which were 3.2 kg hm"2 and 13 kg hm"2, respectively. NCV-N accumulation in soil profile offered the possibility of which NCV-N moved further into the surrounding.The amount of NCV-N in topsoil from nitrogen transformation was determined by the rate mineralization and nitrification successively. The rate of nitrogen mineralization in topsoil in forest was the highest and came to 10.7 kg hm"2 d"1, nevertheless, NCV-N accumulation was low due to the rate of its nitrification only accounting for 1/3 of that of mineralization, namely nearly 1/3 of nitrogen mineralization was merely turned into NCV-N, thereby the risk of environment contamination was low. As far as vegetable greenhouse was concerned, the mineralization, the rate of mineralization and nitrification were all greatly high, and inorganic nitrogen from mineralization could mostly transformed into NCV-N owing to the rate of mineralization and nitrification at the same basically, which was one of important mechanics of NCV-N accumulation in soil in vegetable greenhouse. Although the rate of nitrification in topsoil in grassland was over that of mineralization, the amount of NCV-N from nitrification was low due to the low mineralization. The amount of NCV-N from nitrification in fields with different soil groups would excel that of grassland because the rate of both mineralization and nitrification at the same in the three fields, were higher compared with that of grassland. Therefore, the characteristics of mineralization and nitrification were changed by agricultural practice, the risk of environment contamination increased derived from the great deal of N03"-N yields.Inorganic nitrogenous fertilizer applied for a long term would increase the ratio of inorganic nitrogen, soil microbial biomass N and nonexchangeable ammonium in total N in soil, and meanwhile the ratio of mineralized nitrogen and the potential of mineralization and nitrification decreased in total N in soil. The ratio of several important components of reactive nitrogen pool in topsoil would changed due to inorganic nitrogenous fertilizer applied for a long term in comparison with each other. The turn of different fertilization treatmentsaccording to the ratio of the sum of inorganic nitrogen, soil microbial biomass N and nonexchangeable ammonium in total N from the high to the low was PK>NK>NP>NPK>N>K>CK>P. The result showed that the capacity of reactive nitrogen increased owing to nitrogenous fertilization.Concerning the above results, NCV-N accumulation in soil profile should be reduced through low nitrogenous fertilization, inorganic fertilizer replaced with organic fertilizer and irrigation controlled etc. As far as farmlands were concerned, inorganic nitrogenous fertilizer should go down, especially NO3VN under the condition of the supply of water by precipation in field.